JMMM
Mechanical behavior and physicochemical modifications in lignosulfonate-treated fique ( Furcraea Andina ) fibers
Beatriz Dantas Lourenço da SILVA
1, Paula Lage AGRIZE
1, Betina Carvalho VEIGA
1, Lucio Fabio Cassiano NASCIMENTO
2, Camila Aparecida Abelha ROCHA
1and Fábio de Oliveira BRAGA
1,*1 Universidade Federal Fluminense (UFF), Department of Civil Engineering, Rua Passo da Pátria 156, Bloco D, 4º andar, Niterói, Rio de Janeiro, 24210-240, Brazil
2 Instituto Militar de Engenharia (IME), Department of Materials Science, Praça General Tibúrcio 80, Urca, Rio de Janeiro, 22290-270, Brazil
*Corresponding author e-mail: [email protected] Received date:
1 November 2022 Revised date
1 February 2023 Accepted date:
19 February 2023
Keywords:
Natural lignocellulosic fiber;
Surface treatment;
Physical properties;
Mechanical properties;
Microstructure
Abstract
One of the most frequent problems in biocomposites is the physical/chemical incompatibility between natural lignocellulosic fibers (NLF) and the matrix. Physical and/or chemical treatments are the most common approach to improve interface properties. Seeking for environmentally friendly treatments, lignosulfonates (LSs) have been considered for surface modification of NLF due to their amphiphilic properties. Thus, the objective of the present work is to evaluate the influence of sodium LS (SLS) treatment protocols in the properties and mechanical behavior of fique (Furcraea Andina) fibers. X-ray diffraction (XRD), Thermogravimetric (TGA) and spectroscopic (FTIR) analyzes were performed, as well as optical microscopy, scanning electron microscopy (SEM), fiber-diameter measurements and tensile tests. The results showed efficient surface cleaning from extractives and absorption of saline and aromatic components from SLS. Partial removal of lignin and hemicellulose was observed as well, manifested by an increasing in fiber crystallinity, decreasing in the intensity of the characteristic bands of C=O (acetyl and ester) and p-hydroxy-phenyl, and displacement in the temperature of cellulose pyrolysis to lower temperatures. These phenomena were stronger in the fibers with longer exposures to SLS. The SLS treatment protocols produce a narrower strength distribution, improving the reliability of the fibers relative to its mechanical behavior.
1. Introduction
Over the last 30 years, utilization of natural lignocellulosic fibers (NLF) has been rapidly increasing, due to their interesting properties such as high strength and stiffness, low density and cost, as well wide availability from natural sources [1-4]. These fibers have been left aside due to the advent of the synthetic fibers in the middle part of the twentieth century. However, environmental concerns brought NLF again to attention of scientific community [4-6].
One of the most promising applications for NLF is composite reinforcement. There are several successful cases in engineering and research [1-11]. NLF have shown good results as reinforcement of polymers [1-7], Portland cement and other mineral-based matrices [8-11]. They are known to increase tensile, flexure and impact strength of these materials, as well as maximum deformation, either extension or deflection [1-11].
Biocomposites reinforced with NLF are considered more sustainable than synthetic fiber composites, since their production and processing involve lower energy consumption and net-zero carbon emission in the fiber life cycle [12]. This alternative drives research in the area, seeking to improve the properties of the produced materials.
Fique fibers (Furcraea Andina) are promising for composites application due to their long length and good properties. Colombia native, but also produced in Ecuador, Costa Rica, Antilles and Brazil [13], fique fibers are considered resistant, rigid and with a smooth surface [13-17]. Gañán and Mondragon [13] reported tensile strength of 237 ± 51 MPa, modulus of elasticity of 8.01 ± 1.47 GPa and total elongation of 6.02 ± 0.69% [13,14]. According to Bastidas [14], the dry mass of the raw fique fibers is composed of cellulose (42.1 ± 0.6 wt%), hemicellulose (13 ± 0.4 wt%), lignin (18.2 ± 0.8 wt%), extractives (15.3 ± 0.3 wt%) and ashes (1.1 ± 0.2 wt%). They also reported [14]
that the thermal degradation of the fibers occurs around 360℃. Due to their good properties, several researchers are currently investigating the properties of fique fibers for composite reinforcement [18].
One of the most frequent problems to be faced for biocomposites is the chemical incompatibility between fiber and matrix. Surface physicochemical treatments are the most common approach to improve fiber-matrix interface properties [19-22].
In search of an environmentally friendly treatment, lignosulfonates (LSs) have been considered and applied for surface modification of NLF with interesting results [23-25]. LSs are chemicals produced in the sulfite pulping process of wood, from the breakage of the lignin
polymer chain [26]. According to Ruwoldt [26], there are several types of lignosulfonates, such as sodium, magnesium, calcium, and ammonium. LSs are characterized by their anionic, surfactant, binding, wetting and plasticizing behavior. When wet, LSs develop adhesiveness and agglomeration power. Still according to Ruwoldt [26], LSs have amphiphilic properties, having both hydrophilic and hydrophobic constituents in its molecular structure, and thus can adsorb to several surfaces and interfaces. Anionic groups such as sulfonate and carboxylic ensure water solubility, while less polar groups such as aromatic and aliphatic chains facilitate interaction with organic surfaces and interfaces [26].
The use of LSs as NLF surface treatment agents has been attempted with good results for modification of natural fibers such as sisal and wool [23-25]. Jose et al. [23] studied the modification of wool fabric with sodium LS (SLS) to improve the interfacial adhesion of natural rubber latex in the wool/rubber composites. They found out that the SLS acted as a cementing material between the wool fiber and latex, resulting in improved composite properties, such as better elongation and retention of tensile strength after UV ageing [23]. Oliveira et al.
[24] studied the modification of sisal fibers with SLS to improve the properties of phenolic-matrix composites. The results showed greater impact strength of the composites reinforced with SLS-modified sisal fibers. Gualberto et al. [25] studied the treatment of sisal fibers with SLS to improve the properties of cement-matrix composites.
The resulting composites showed better tenacity and better fiber- matrix adhesion. However, the action of the LSs in lignocellulosic fibers needs to be further studied, and currently there is no investigation that evaluates the influence of LSs in the structure and properties of fique (Furcraea Andina) fibers. Therefore, the objective of the present work is to evaluate mechanical, physical, and chemical changes in the fique fibers when subjected to several treatment protocols using SLS. Thus, the present work evaluates for the first time the physico- chemical modifications of the fique fibers subjected to specific SLS treatment protocols and might be important to understand further studies on fiber-matrix adhesion in composites.
2. Experimental
2.1 Materials 2.1.1 Fique fibersFor the development of this work, fique fibers (Furcraea Andina) (Figure 1) were acquired from local producers in Colombia. The fibers were manually cleaned with a nylon hairbrush, to remove some dry wax and to individualize (separate) them. The fibers were then cut to the lengths specified to each test.
2.1.2 Sodium lignosulfonate (SLS)
Sodium lignosulfonate (SLS) was used as modification agent of the fibers. The SLS powder was provided by Lignotech Brasil (Brazil), from the Borregaard Lignotech group, type 1259 Ultrazine NA (CAS No. 8061-51-6). According to the manufacturer, Ultrazine NA is based on highly refined and modified sodium lignosulfonate derived from spruce wood and sulfite liquor. It was provided as a brown water- soluble powder. Table 1 show some basic specifications of the chemical.
Figure 1. General aspect of the fique fibers.
Table 1. Basic Specifications of the Borregaard Ultrazine NA.
Sales specifications
pH (10% solution) 8.7 ± 0.8
Insolubles (%w/w) Max. 0.1
Sodium % 7
Calcium % Max. 0.1
Reducing sugars % 1
Chloride % Max. 0.10
Alkali % Max. 12%
Table 2. Specimen designation.
Treatment protocol Specimen designation
1h immersion in water at 70℃ Raw fique 1h immersion in 5 wt% SLS solution at 70℃ FL-70-1h 24h immersion in 5 wt% SLS solution at 25℃ FL-24h 1h immersion in 5 wt% SLS solution at 25℃
in the ultrasonic bath FL-1h-US
2.2 Methods 2.2.1 Fiber processing
The dry and cut fibers were separated into 4 groups, 3 for the SLS treatment protocols and 1 for the control group. The fibers from the control group were immersed in 70℃ deionized water for 1 h, and then dried in the oven at 104 ± 2℃ until constant mass. The fibers from this group were designated as Raw Fique (Table 2). The SLS treatment protocols were carried out by immersing the fibers in a 5 wt% SLS solution in deionized water. These treatment protocols were previously employed to other natural fibers by other researchers [24,25], with good results.
The first SLS treatment was very similar to the control group, however, the fibers were immersed in the 5 wt% SLS solution (at 70℃ for 1 h) instead of pure deionized water. The fibers from this group were designated as FL-70-1h (Table 2). In the second SLS treatment, the fibers were immersed in the 5 wt% SLS solution for 24 h at room temperature (25℃). The fibers from this group were designated as FL-24h (Table 2). In the third treatment, the fibers were immersed in the 5 wt% SLS solution for 1h subjected to ultrasonic irradiation (40 kHz), in a laboratory ultrasonic bath, model SoniClean 2PS (Sanders). The fibers from this group were designated as FL-1h-US (Table 2). After each treatment, the fibers were dried in an oven until reaching constant mass.
The objective of using ultrasonic energy was to increase the surface free energy of the fibers, to make the surface more active and receptive to the SLS molecules. The ultrasonic vibration is known to cause cavitation, a physicochemical phenomenon of formation and outbreak of bubbles [24].
2.2.2 Optical microscopy and diameter measuring
The optical micrographs of the fibers were obtained in a model LAB-60T Laborana stereomicroscope. The diameter measuring of the fibers was performed in a model BX3M Olympus optical microscope.
The fibers were positioned one by one, stretched, fixed on a cardboard fillet, and observed at 100x magnification. Thirty-five fibers from each treatment were analyzed. The images were captured by a camera attached to the microscope, and the diameters were measured at three different points along the length, using LCmicro software. The data was treated using Analysis of Variance (ANOVA) to evaluate the statistical significance of any diameter variation.
2.2.3 Scanning Electron Microscopy (SEM)
The SEM analyzes were performed in a model Quanta FEG 250 FEI scanning electron microscope, to evaluate the modifications in the surface of the fibers. Secondary electrons contrast was used to observe the surface morphology, as well as Energy-dispersive X-ray spectroscopy (EDS) analysis to correlate the images to their chemical composition.
2.2.4 X-Ray Diffraction (XRD)
The X-ray diffraction analyzes were performed to evaluate the changes in the crystal structure and degree of crystallinity of the fibers, resulting from the SLS treatments. A model D8 Advance Polycrystal Bruker diffractometer was used. Intensity measurements were performed in the 2θ range between 10° and 70°, with a 0.05°
step and 2°⸳min-1 scanning velocity. As sample preparation, the fibers were cut with scissors to lengths of less than 1 mm. To estimate the Crystallinity Index (Xc) of the raw and treated fique fibers, the diffraction patterns were processed using Origin 8 software, to obtain data on the peaks of cellulose and amorphous phases. The crystallinity index (Xc) is calculated from Equation (1) [27,28]:
𝑋𝑋𝑐𝑐= 𝐼𝐼002𝐼𝐼−𝐼𝐼𝑎𝑎𝑎𝑎
002 ∙ 100% (1)
Where: Xc is the fiber crystallinity index; I002 is the maximum intensity (in arbitrary units) of the (002) lattice diffraction peak (2θ around 22°); Iam is the height of the minimum between the (002) and the (101) peaks (minimum at 2θ around 18.5°).
2.2.5 Fourier transform infrared spectroscopy (FTIR) The fibers were analyzed by means of Fourier transform infrared spectroscopy (FTIR), in the Attenuated Total Reflection (ATR) mode, to evaluate chemical changes resulting from the SLS treatment protocols.
The spectra were obtained using a model Spectrum 100 Perkin Elmer infrared spectrometer, in the range of 4000 cm-1 to 650 cm-1. The spectra were analyzed following ASTM D2224 standard [29].
2.2.6 Thermogravimetric analysis (TGA)
The thermogravimetric analysis (TGA) was performed to evaluate the thermal stability of the fibers before and after the SLS treatments.
The analyzes were performed in a model DTG-60 Shimadzu equipment, from ambient temperature (21℃) to 500℃, at a heating rate of 10℃⸳min-1, in a nitrogen atmosphere. The spectra were analyzed following ASTM E2550 standard [30].
2.2.7 Tensile tests
Tensile tests of the raw and treated fibers were carried out following the procedures of the ASTM C1557 standard [31], using a model Tytron 250 MTS testing machine. 12 samples were tested for each treatment, with a test speed of 0.3 mm⸳min-1, a 5 kN load cell and a working length of 40 mm. The experimental setup for the tensile test is shown at Figure 2.
Due to specimen variability, the test results were statistically treated by fitting the data to a 2-parameter Weibull distribution [32], characterized by a shape parameter (β) and a scale parameter (θ), as well as the adjustment R². The Weibull cumulative distribution function is given by Equation (2):
𝐹𝐹(𝑥𝑥) = 1− 𝑒𝑒�−�𝜃𝜃𝑥𝑥�
𝛽𝛽� (2)
Figure 2. Experimental setup for the tensile test of the fibers.
(b) (a)
3. Results and discussion
3.1 Optical microscopy and diameter measuring Figure 3 shows the optical micrographs of the untreated (Figure 3(a)) and treated fibers (Figure 3(b-d). The fibers are composed of microfibrils oriented along the fiber axis, with specific features depending on the treatment. The untreated fiber (Raw fique, Figure 3(a)) presents dark marks on the surface, which are probably related to impurities, such as waxes and other extractives. These dark marks could not be observed on the treated fibers (Figure 3(b-d), which show cleaner surfaces, especially the FL-70-1h specimen. Figure 3(b-d) show, also, bright marks spread through the surface, which are probably SLS crystals.
Figure 3. Optical micrographs of the fique fibers, captured by the LAB-60T Laborana stereomicroscope: (a) Raw fique; (b) FL-70-1h; (c) FL-24h; (d) FL-1h-US.
Figure 4. Diameter distributions for the fibers: (a) Raw fique; (b) FL-70-1h;
(c) FL-24h; (d) FL-1h-US.
Table 3. Fiber diameter (d) data.
Specimen dmax (μm) dmin (μm) dm (μm) σ (μm)
Raw fique 123 56 84 15
FL-70-1h 168 61 103 24
FL-24h 202 69 105 28
FL-1h-US 149 59 106 24
Figure 4 shows histograms containing the diameter distributions for the fibers. The most frequent diameters are included in the interval from ~80 and 120 μm. Table 3 shows the minimum (dmin), maximum (dmax), and average (dm) diameter values, together with the standard deviation (σ) of the distributions. Diameters greater than 125 μm were only found among the treated fibers, and minimum values were slightly greater than the observed values for the Raw fique fibers. However, the Analysis of Variance (ANOVA) indicated that the average values should be considered statistically similar (p-value 0.976888), considering significance level of 5%. Differences in the average diameters were not expected, since the result of the treatment protocols would be to provide a thin layer of SLS and crystallization products on the fibers surface.
3.2 Scanning Electron Microscopy (SEM)
Figure 5 to Figure 8 show the SEM micrographs of the fibers.
Figure 5 confirms that Raw fique fibers present extractives on the surface, which might be removed by the treatment protocols. The EDS analysis showed presence of carbon (C), oxygen (O) and calcium (Ca).
As widely known, C and O are the basic elements of the lignocellulosic structure. About Ca, Ovalle-Serrano [16] reported that the element is useful to the plant to structurally preserve the cell wall. Figure 6 shows that the FL-70-1h treatment was able to remove these impurities.
Besides C, O and Ca, traces of sodium (Na) were identified by the EDS analysis, indicating absorption of SLS components.
Figure 7 shows that the FL-24h treatment protocol was also able to remove the extractives, however, intense precipitation of SLS crystallization products was observed. As for FL-70-1h treatment, EDS analysis of the FL-24h surface also shows C, O, Ca and traces of Na.
Figure 8 shows that FL-1h-US treatment is also efficient to remove the impurities, and, unlike FL-70-1h and FL-24h, no traces of Na were identified, so only C, O and Ca were found.
Figure 5. SEM micrographs of the Raw fique fibers: (a) 1,000x, and (b) 5,000x.
Figure 6. SEM micrographs of the FL-70-1h: (a) 1,000x, and (b) 5,000x.
Figure 7. SEM micrographs of the FL-24h: (a) 1,000x, and (b) 5,000x.
Figure 8. SEM micrographs of the FL-1h-US: (a) 1,600x,and (b) 5,000x.
3.3 X-Ray diffraction (XRD)
Figure 9 shows the diffraction patterns for fique fibers. Typical peaks for the native cellulose (ICDD database, PDF 03-0289) are present, as summarized in Table 4 [16,33]. The small peaks at 30.6º and 38.7º (Figure 9) were not included in Table 4, since they were not reported in the aforementioned references [16,33], however, they were also identified as native cellulose, according to the PDF 03-0289 file.
The crystallinity index (Xc) has been calculated from the diffraction patterns (Figure 9) and the data are presented in Table 5. The obtained Xc values (Table 5) are coherent with literature values [16] and are consistent with the common cellulose content around 30% to 50 % [14, 16]. Since cellulose is considered the only relevant crystalline compound in the NLFs, Xc should follow the values of cellulose content. Ovalle-Serrano [16], for example, obtained Xc = 63% for raw fique fibers, Xc = 57% for fique tow and Xc = 35% for fique pulp.
These values show correlation with the cellulose content of those specimens, 52.9 ± 2.1%, 52.3 ± 3.1% and 31.5 ± 1.1%, respectively.
This is an indication that increasing cellulose content due to lignin and hemicellulose dissolution would explain the variation of Xc. In the present work, the SLS increased the crystallinity of the fique fibers for all treatment protocols. Also, longer treatment times, such as in FL-24h specimen, could favor the phenomenon of lignin, hemicellulose and other extractives dissolution, increasing the resulting crystallinity.
3.4 Fourier-transform infrared spectroscopy (FTIR) Figure 10 shows the FTIR spectra for the several specimens.
Figure 10 shows that typical lignocellulosic groups are present in all specimens. 3341 cm-1 absorption band is attributed to OH stretching due to phenolic and open alcoholic groups. 2923 cm-1 band is attributed
to C-H stretching due to methyl and other alkyl groups. 1733 cm-1 band is attributed to C=O stretching vibration in acetyl and ester groups in lignin, hemicellulose and pectin [22]. 1604 cm-1 is attributed to C=C vibration due to aromatic rings in lignin [22,34]. 1026 cm-1 band is attributed to C-O stretching and C-H vibration, both present in the lignocellulosic compounds (cellulose, hemicellulose and lignin).
895 cm-1 band is associated with the cellulosic β-glycosidic linkages [16,22]. 663 cm-1 is attributed to S-O stretching in sulfonic groups, which, according to Jose et al. [23] results from the response of sodium sulfite with the secondary hydroxyl of the aliphatic side chain of lignin.
Figure 9. X-ray diffraction patterns for the raw and treated fibers.
Figure 10. FTIR spectra for the raw and treated fibers.
Table 4. Peak information from the XRD spectra of the raw and treated fique fibers.
Peak position (degrees)
Structure Crystalline plane
15.0 Cellulose I (110) [16,33]
16.5 Cellulose I (1Ī0) [33]
22.5 Cellulose I (200) [16,33]
35.0 Cellulose I (004) [16,33]
Table 5. Peak intensity data and calculated crystallinity index (Xc).
Specimen I002 Iam Xc (%)
Raw fique 9453 6744 28.7
FL-70-1h 7266 4053 44.2
FL-24h 4362 2074 52.4
FL-1h-US 8494 5363 36.9
Analyzing the spectra (Figure 10), it can be noticed that the FL-70-1h spectrum is the most similar to the untreated fiber (Raw fique), which indicates that the treatment with SLS for 1h at 70℃ had little effect in the chemistry of the fique fibers. Apart from that, the main differences between the spectra are detailed below: (a) 1733 cm-1 (C=O) band is less intense for FL-24h and FL-70-1h spectra, indicating a decrease of C=O related groups such as acetyl and ester groups. This has been observed by Gañan and Mondragon [13] for the mercerization process of a lignocellulosic fiber that resulted in removal of some lignin (and hemicellulose); (b) 831 cm-1 band is absent in the FL-24h spectrum, which is an indication of breaking of the p-hydroxy-phenyl groups in lignin, also compatible to mercerization process in alkaline medium.
It is important to mention that SLS solutions are alkaline, and their pH is around 8.7, according to the supplier’s datasheet; (c) The band in 775 cm-1 has a higher intensity in the FL-24h spectrum, which, together with the 895 cm-1 band shoulder, indicates the absorption of saline compounds from SLS by the fibers [35,36]. The presence of the 1230 band (S=O) corroborates this information, although this band might be overlapped with C-C and C-O group absorptions [33-36]; (d) Bands at 1604 cm-1 and 1508 cm-1 (C=C) are more intense for FL-24h, and a little less for FL-1h-US. These are related to vibration of the aromatic skeleton of lignin [13,37]. These absorptions together with the 1260 cm-1 (guaiacyl) in FL-24h, suggests that, although p-hydroxy-phenyl groups in lignin are decreasing (partial degradation of lignin in the fiber), other groups such as guaiacyl are increasing, probably due to introduction of lignin-related groups from SLS.
3.5 Thermogravimetric analysis (TGA)
Figure 11 shows the thermogravimetric (TG) curves for the raw and treated fibers. All specimens presented three specific features:
(a) a small mass drop (~10 %) below 100℃ due to vaporization of moisture; (b) an intense weight loss (50% to 70%) in the 250℃ to 350℃ temperature range; (c) a smooth and continuous mass loss over 350℃. According to several authors [13,14,16,24], (b) is due to thermal depolymerization of hemicellulose (~250℃) and pyrolysis of cellulose (~350℃). Figure 12 shows these two processes for the Raw fique, by the separation of the two reaction minima in the DTG curve.
The weight losses in (b) are consistent to the lignocellulosic composition observed by Bastidas [14]. According to the authors [14], cellulose and hemicellulose contents in the fique fibers were 42.1 ± 0.6 wt% and 13 ± 0.4 wt%, respectively, which would totalize a weight loss around 55%. This is consistent to the information on Figure 11, which shows a 50% to 70% weight loss. Partial degradation of lignin could occur simultaneously to hemicellulose and cellulose, which would complete the degradation process.
The disappearing of one reaction minimum on the DTG curves (Figure 12) for the treated specimens (FL-70-1h, FL-24h and FL-1h-US)
is an important feature. This could be explained by the suppression of the hemicellulose degradation at 300℃, due to the removal of hemicellulose by the SLS treatments. The thermal event at 300℃
is probably due to cellulose degradation, which has been displaced to lower temperatures, since cellulose became more susceptible to thermal degradation, as observed by Oliveira et al. [24]. The variation on the residual mass is also consistent with the phenomenon of removal of hemicellulose/lignin. FL-24h and FL-1h-US were the specimens of higher residual mass, indicating that a higher content of extractives has been removed during the treatments.
The reactions at 400℃ and 450℃ are only observed for the treated fibers and could be related to decomposition of aromatic rings of the SLS [24].
Figure 11. TG curves for the raw and treated fibers.
Figure 12. DTG curves for the raw and treated fibers.
Table 6. Tensile test data obtained from the stress-strain curves.
Specimen Strength Maximum strain
β (MPa) θ (MPa) R² β (%) θ (%) R²
Raw fique 1.31 283.44 0.92 2.09 10.33 0.74
FL-70-1h 1.46 167.51 0.93 2.13 14.07 0.90
FL-24h 2.38 159.24 0.94 2.57 14.20 0.98
FL-1h-US 2.33 147.46 0.94 1.95 11.58 0.90
3.6 Tensile tests
Figure 13 shows the tensile stress-strain curves for the fibers.
The curves in Figure 13 show typical elastic stress-strain behavior until fracture and a broad variability of values for strength and maximum strain, attributed to diameter variation and other known heterogeneities in the fibers [38]. Table 6 show the Weibull statistical parameters applied to strength and maximum strain. The scale parameter (θ) represents the approximate center of the distribution, the value under which 63.2% of the fibers will fail, while the shape
parameter (β) represents the variability of the data. The larger β, the narrower is the distribution. Figure 14 shows the distribution plots obtained from the parameters in Table 6. The results show that all treatments, especially FL-24h and FL-1h-US, produces a narrower strength distribution, improving the reliability of the fibers relative to its mechanical behavior. This is probably due to the partial removal of hemicellulose/lignin, which decreases the number of critical defects on the fibers. On the other hand, the treatments do not seem to influence the maximum deformation.
Figure 13. Tensile stress-strain curves for the fibers: (a) raw fique; (b) FL-70-1h; (c) FL-24h; (d) FL-1h-US.
Figure 14. Weibull density of probability function for (a) strength; (b) maximum extension.
4. Conclusions
In the present work, several treatment protocols using sodium lignosulfonate (SLS) were applied to fique (Furcraea Andina) fibers, to evaluate the mechanical, physical, and chemical changes. The treatment protocols were: (1) Raw fique (control group): 1-h immersion in deionized water at 70℃; (2) FL-70-1h: 1-h immersion in 5 wt%
SLS solution at 70℃; (3) FL-24h: 24-h immersion in 5 wt% SLS solution at room temperature (25℃); (4) FL-1h-US: 1-h immersion in 5 wt% SLS solution in conjunction with ultrasonic irradiation (40 kHz). The main conclusions are presented below:
• All evaluated SLS treatments protocols were efficient in cleaning the fiber surface from extractives, however, caused deposition of SLS products over the fiber surface, especially the FL-24h, which produced intense deposition of crystallization products.
• SLS treatment protocols did not significantly change the fibers average diameter. The most frequent observed diameters were included in the range from ~80 μm and 120 μm.
• SLS treatment protocols increased the crystallinity of fique fibers due to lignin, hemicellulose, and other extractives dissolution.
The treatment protocol with higher influence on the fiber crystallinity was the FL-24h.
• FL-70-1h treatment did not significantly affect the fiber chemistry. On the other hand, the treatment with higher influence in the fiber chemistry was the FL-24h, which caused a decreasing in the amount of C=O related groups (acetyl and ester groups) and breaking of the p-hydroxy-phenyl groups in lignin, due to lignin (and hemicellulose) removal. Absorption of saline and aromatic compounds from SLS was observed in all treated specimens.
• Cellulose pyrolysis has been displaced to lower temperatures for all treated specimens, indicating that cellulose became more susceptible to thermal degradation. Thermal reactions around 400℃
and 450℃ were observed for the treated fibers, probably due to decomposition of aromatic rings of the deposited SLS.
• The tensile behavior of the untreated and treated fibers was characterized by typical elastic stress-strain behavior until fracture and a broad variability of values for strength and maximum strain, due to diameter variation and other heterogeneities in the fibers.
The SLS treatment protocols produce a narrower strength distribution, improving the reliability of the fibers relative to its mechanical behavior, probably due to the (partial) removal of hemicellulose/
lignin, which decreases the number of critical defects.
Acknowledgements
The authors of the present work would like to acknowledge:
FAPERJ for funding the project in the Basic Research Aid (APQ1) Program, process 211.544/2021; UFF/Brazil Research Funding Program (FOPESQ 2021), for funding the project; PPGCM/IME/Brazil for the donation of the fique fibers, as well as enabling the diameter measuring, SEM and FTIR analyzes; NUMATS/COPPE/UFRJ/Brazil for performing the tensile tests; LDRX/IF/UFF/Brazil for performing the XRD tests; RECAT/UFF/Brazil for performing the thermo- gravimetric analyzes; Lignotech Brasil, for the donation of the lignosulfonate.
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